The present invention relates to a linear transport apparatus for transporting articles by driving a trough by means of a drive unit reciprocatingly in directions opposite to each other along a path of transport of articles placed on the trough.
Hitherto, linear transport apparatuses have been well known in the art for transporting articles by driving a trough reciprocatingly in directions opposite to each other along a path of transport of articles placed on the trough The linear transport apparatus is also known of a type wherein a linear motor (a drive unit) including a magnet and a stator comprised of an iron core having a primary winding wound therearound is used with the stator mounted on a base while the magnet is secured to an undersurface of the trough, on which articles to be transported are placed, in a manner spaced a predetermined distance from the stator, so that the trough can be reciprocatingly driven by the linear motor along a predetermined transport path to thereby transport the articles on the trough. See the Japanese Patent Publication No. 54-35395.
The operation of the above described linear transport apparatus to transport the articles is such that while at the outset the articles are placed on the trough, an electric power is supplied to a coil to activate the drive unit and the trough is moved in a forward direction at a advancing speed having a peak value generally insufficient to overcome a static frictional resistance (a static frictional force) of the articles relative to the trough. By so doing, the articles are moved in the forward direction together with the trough. Then, the drive unit is reversed to allow the trough to be moved in a rearward direction at a retracting speed having a peak value higher than the advancing speed so that the peak value can overcome the static frictional resistance of the articles relative to the trough. By so doing, the articles can slide relatively on the trough in the forward direction while bearing a kinetic frictional resistance (a kinetic frictional force) that is smaller than the static frictional resistance. The foregoing reciprocating movement of the trough is cyclically repeated to allow the articles to be successively transported along the direction of transport thereof.
During the relative movement between the trough and the articles in the manner described above, the kinetic frictional force F acts at respective surface areas of contact between the trough and the articles, which force F has a relationship of F=xcexcN wherein xcexc represents a coefficient of kinetic friction and N represents a force acting in a direction normal to the surface areas (a normal force). Specifically, the coefficient of kinetic friction xcexc means a ratio between the kinetic frictional force F, induced at the interface between the trough and the articles then held in a state of kinetic friction during which the articles undergo movement relative to the trough, and the normal force N. In such case, the coefficient of kinetic friction xcexc is smaller than the coefficient of static friction xcexc0. Where the articles are to be transported by reciprocatingly driving the trough, with the articles placed thereon, in the direction of transport of the articles, the coefficient of kinetic friction xcexc between the trough and the articles considerably affects the behavior of transport of the articles. Accordingly, it is required to obtain accurate value of coefficient of kinetic friction xcexc.
However, with the prior art linear transport apparatus, it has been a practice to determine how to set a driving characteristic of the linear motor in dependence on a combination of the trough and the articles, that is, to determine how to drive the trough in reliance on the sense of operator or on the seat-of-the-pants of an experienced worker, and therefore, it has been difficult to control the apparatus so that the articles can be transported at a desired transport speed or in a desired transport quantity.
However, in the linear transport apparatus, it has hitherto been difficult to secure accurately the coefficient of kinetic friction xcexc between a desired trough and articles to be transported, and where the number of combinations of the troughs and the articles is great, the difficulty increases correspondingly. As a result, it has been still difficult to control the apparatus so that the articles can be transported at a desired transport speed or in a desired transport quantity.
The present invention has therefore been devised with the foregoing problems taken into consideration and is intended to provide a linear transport apparatus and a method of controlling the same, wherein the articles can easily be transported at a desired transport speed.
In order to accomplish the foregoing object, one aspect of the present invention provides transportation of articles by driving a trough by means of a drive unit reciprocatingly in directions opposite to each other along a path of transport of articles placed on the trough, wherein a plurality of trough displacement characteristics, each descriptive of a relationship between quantity of displacement of the trough and time, and drive signals for driving the drive unit are determined in association with each other, a position of the articles in a direction of transport thereof, which articles are transported by the trough having one of the trough displacement characteristics, is subsequently determined by using a given amplitude of the trough or a given magnitude of the trough stroke and a given coefficient of kinetic friction between the trough and the articles, a speed of transport of the articles is determined according to a time dependent change of the determined position of the articles, and one of the drive signals that is required to render the determined speed of transport of the article to attain a target speed is finally selected.
According to the foregoing construction, since the plural trough displacement characteristics and the drive signal for driving the drive unit of the trough are determined in association with each other and since the speed of transport of the articles transported by the trough having one of the trough displacement characteristics can therefore be determined, a plurality of relationships between the speed of transport of the articles and the drive signal for the drive unit can be determined and, based on these relationships, merely by inputting the target speed of the articles, the drive signal for the drive unit can easily be selected that is required for the speed of transport of the articles to attain the target speed.
Preferably, the position of the articles in the direction of transport thereof is determined by solving an equation of motion of the articles transported by the trough having such one of the trough displacement characteristics by utilization of the given amplitude of the trough and the given coefficient of kinetic friction between the trough and the articles. By so doing, the position of the articles with respect to the direction of transport can easily be determined by the use of the equation of motion of the articles transported by the trough.
In a preferred embodiment of the present invention, the quantity of transport of the articles is determined from a quantity of travel of the articles based on the determined speed of transport of the article and, in place of the selection of one of the drive signals that is required to render the determined speed of transport of the article to attain the target speed, one of the drive signals that is required to render the determined quantity of transport of the articles to attain a target transport quantity is selected.
According to this construction, since the determined quantity of transport of the articles can be determined based on the speed of transport of the articles, and in view of the relationship between the quantity of transport of the articles and the drive signal for the drive unit in dependence on the relationship between the speed of transport of the articles and the drive signal for the drive unit, mere inputting of the target quantity of transport of the article can facilitate an easy selection of the drive signal for the drive unit that is required to render the quantity of transport of the articles to attain the target quantity of transport.
Preferably, a plurality of trough speed characteristics are determined, each characteristics descriptive of a relationship between a trough displacement speed and time when parameters including the amplitude of the trough and an acceleration time of the trough during a forward transport path are changed, and the trough displacement speed is integrated with time to thereby determine the plurality of the trough displacement characteristics each descriptive of a relationship between the trough displacement quantity and the time. Accordingly, the trough displacement characteristics can easily be determined from the trough speed characteristics.
According to a second aspect of the present invention, while the articles are transported by driving a trough by means of a linear transport apparatus reciprocatingly in directions opposite to each other along a path of transport of articles placed on the trough, the coefficient of kinetic friction between the trough and the articles is determined, wherein the quantity of movement of the articles, transported by the trough, relative to the length of time during which the trough is driven is determined with the use of a plurality of coefficients of kinetic friction, induced between the trough and the articles, as a parameter, and a plurality of trough displacement characteristics, each descriptive of a relationship between a quantity of displacement of the trough and time, in association with drive signals for driving the drive unit are stored, and while the trough is actually driven for a predetermined time to transport the articles, the quantity of movement of the articles transported is measured so that the coefficient of kinetic friction required for the quantity of movement stored becomes equal to the quantity of movement measured can be determined from the displacement quantity characteristic.
According to this construction, since the displacement quantity characteristic is determined which is descriptive of a correspondence between the trough driving time and the quantity of movement of the articles with respect to each of the parameters of the plural coefficients of kinetic friction, the quantity of movement of the articles relative to the trough driving time with the plural coefficients of kinetic friction taken as the respective parameters can be grasped and, therefore, based on this a desired coefficient of kinetic friction can easily be obtained from the quantity of movement of the articles measured by actually driving the trough for the predetermined time to transport the articles.
The quantity of movement of the articles relative to the trough driving time can be obtained by solving the equation of motion of the articles transported by the trough, in which the plural coefficients of kinetic frictions given between the trough and the articles are taken as a parameter.
Preferably, in determining the above described quantity of movement, the trough is driven reciprocatingly under the same drive mode for each of the plural parameters. Accordingly, only one mode is sufficient for the drive mode to be used during actual measurement of the quantity of movement of the articles and, therefore, the coefficients of kinetic friction between the trough and the articles can further easily be determined.